Data retention structure for non-volatile memory

ABSTRACT

A data retention structure in a memory element that stores data as a plurality of conductivity profiles is disclosed. The memory element can be used in a variety of electrical systems and includes a conductive oxide layer, an ion impeding layer, and an electrolytic tunnel barrier layer. A write voltage applied across the memory element causes a portion of the mobile ions to move from the conductive oxide layer, through the ion impeding layer, and into the electrolytic tunnel barrier layer thereby changing a conductivity of the memory element, or the write voltage causes a quantity of the mobile ions to move from the electrolytic tunnel barrier layer, through the ion impeding layer, and back into the conductive oxide layer. The ion impeding layer is operative to substantially stop mobile ion movement when a voltage that is less than the write voltage is applied across the memory element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 12/075,017, filed Mar. 7, 2008, the disclosure of which isherein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to non-volatile memory. Morespecifically, the present invention relates to thin-film structures innon-volatile memory.

BACKGROUND OF THE INVENTION

Data storage in high-density memory devices can be accomplished using avariety of techniques. Often, the technique used depends on whether ornot the stored data is volatile or non-volatile. In volatile memorydevices, such as SRAM and DRAM, for example, stored data is not retainedwhen power is removed from the memory device. On the other hand, fornon-volatile memory devices, such as MRAM and Flash devices, stored datais retained when power is removed from the memory device.

Resistive state memory devices are a promising new type of non-volatilememory in which data is stored in a memory element as a plurality ofconductivity profiles (e.g., distinct resistive states). A firstconductivity profile can represent a logic “1” and a second conductivityprofile can represent a logic “0”. The first and second conductivityprofiles can be set by applying a write voltage of a predeterminedmagnitude, polarity, and duration across the memory element during awrite operation. For example, voltage pulses can be used to write alogic “1” and a logic “0”, respectively.

In either case, after data has been written to the memory element,reading the value of the stored data in the memory element is typicallyaccomplished by applying a read voltage across the memory element andsensing a read current that flows through the memory element. Forexample, if a logic “0” represents a high resistance and a logic “1”represents a low resistance, then for a constant read voltage, amagnitude of the read current can be indicative of the resistive stateof the memory element. Therefore, based on Ohm's law, the read currentwill be low if the data stored is a logic “0” (e.g., high resistance) orthe read current will be high if the data stored is a logic “1” (e.g.,low resistance). Consequently, the value of the stored data can bedetermined by sensing the magnitude of the read current.

In high density memory devices, it is desirable to pack as many memorycells as possible in the smallest area possible in order to increasememory density and data storage capacity. One factor that can have asignificant impact on memory density is the number of terminals that arerequired to access a memory element for reading or writing. As thenumber of terminals required to access the memory element increases,device area increases with a concomitant decrease in areal density. Mostmemory technologies, such as DRAM, SRAM, and some MRAM devices, requireat least three terminals to access the core memory element that storesthe data. However, in some memory technologies, such as certainresistance based memories, two terminals can be used to both read andwrite data to/from the memory element.

An array of two terminal memory elements can include a plurality of rowconductors and a plurality of column conductors and each memory elementcan have a terminal connected with one of row conductors and the otherterminal connected with one of the column conductors. The typicalarrangement is a two terminal cross-point memory array where each memoryelement is positioned approximately at an intersection of one of the rowconductors with one of the column conductors. The terminals of thememory element connect with the row and column conductors above andbelow it. A single memory element can be written by applying the writevoltage across the row and column conductors the memory element isconnected with. Similarly, the memory element can be read by applyingthe read voltage across the row and column conductors the memory elementis connected with. The read current can be sensed (e.g., measured)flowing through the row conductor or the column conductor.

One challenge for some non-volatile memories is data retention, that is,the ability of stored data to be retained in the absence of power.Ideally, stored data is retained indefinitely in the absence of power.Examples of factors affecting data retention include but are not limitedto memory element structure, material used in the memory element, andvoltages applied across the memory elements during data operations, suchas read and write operations. When a read or write voltage is appliedacross the two terminals of a selected memory element, approximatelyhalf of the voltage potential is supplied by the row conductor and halfby the column conductor. Accordingly, other memory elements having aterminal connected with the row conductor or column conductor also havea voltage potential applied across their respective terminals. Thoseun-selected memory elements are generally referred to as half-selectedmemory elements because one of their terminals has ½ of a read voltagepotential or ½ of a write voltage potential applied to it and the otherterminal is typically at a ground potential. The potential differenceacross the terminals is referred to as a half-select voltage. Thehalf-select voltage can generate electric fields, that over time, candisturb (e.g., corrupt) the stored data in those memory elements.Moreover, because write voltages are typically greater in magnitude thanread voltages, the half-select voltages during write operations aregreater than the half-select voltages during read operations. Therefore,it is desirable for the write voltages to affect stored data only in theselected memory element(s) and not in half-selected memory elements.

Although the magnitude of half-select voltages may be lower for readoperations, in some applications, a majority of data operations to anon-volatile memory may comprise read operations. Repeated readoperations may result in numerous applications of read voltages andhalf-select voltages to memory elements in a memory device. Theapplication of half-select voltages during read operation may affectdata retention in half-selected memory elements. However, those skilledin the art will appreciate that some design choices will affect theextent an array is exposed to half-select voltages. For example, a pagemode read might not cause the array to experience any half-selectvoltages during read operations.

There are continuing efforts to improve non-volatile memory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a memory element including mobile ions and storing dataas a first conductivity profile;

FIG. 2A depicts a memory element switching from a first conductivityprofile to a second conductivity profile;

FIG. 2B depicts a memory element having the second conductivity profile;

FIG. 2C depicts retention of the second conductivity profile;

FIG. 3A depicts a memory element switching from the second conductivityprofile to the first conductivity profile;

FIG. 3B depicts a memory element having the first conductivity profile;

FIG. 3C depicts retention of the first conductivity profile;

FIG. 4A depicts a memory element having a second conductivity profilethat is unaffected by application of a first read voltage;

FIG. 4B depicts a memory element having a first conductivity profilethat is unaffected by application of the first read voltage;

FIG. 4C depicts a memory element having a second conductivity profilethat is unaffected by application of a second read voltage;

FIG. 4D depicts a memory element having a first conductivity profilethat is unaffected by application of the second read voltage;

FIG. 5 depicts a memory element electrically in series with andsandwiched by a pair of electrodes;

FIG. 6A depicts a non-ohmic device and a memory element that areelectrically in series with and sandwiched between a pair of electrodes;

FIG. 6B depicts an alternate configuration of a non-ohmic device and amemory element that are electrically in series with and sandwichedbetween a pair of electrodes;

FIG. 7A depicts a portion of a non-volatile two-terminal cross-pointarray including a non-volatile memory plug electrically in series with afirst conductive array line and a second conductive array line;

FIG. 7B depicts a schematic view of a non-volatile two-terminalcross-point array that includes a plurality of memory plugs;

FIG. 7C depicts selected, half-selected, and un-selected memory plugs ina non-volatile two-terminal cross-point array;

FIG. 8A is a cross-sectional view depicting a non-volatile two-terminalcross-point array positioned over a substrate that includes activecircuitry;

FIG. 8B is a cross-sectional view depicting a stacked non-volatiletwo-terminal cross-point array positioned over a substrate that includesactive circuitry;

FIG. 9 is a table depicting data for erase and program slopes for memoryelements with and without ion impeding layers;

FIG. 10 is a plot depicting current loss over time for memory elementswith and without ion impeding layers;

FIG. 11 depicts a memory system including a non-volatile two-terminalcross-point array; and

FIG. 12 depicts an exemplary electrical system that includes at leastone non-volatile two-terminal cross-point array with a data retentionstructure for data storage.

Although the previous drawings depict various examples of the invention,the invention is not limited by the depicted examples. Furthermore, thedepictions are not necessarily to scale.

DETAILED DESCRIPTION

In the following detailed description and in the several figures of thedrawings, like elements are identified with like reference numerals.

As shown in the drawings for purpose of illustration, the presentinvention is embodied in a non-volatile memory device, a non-volatilememory element, and a non-volatile memory array.

Reference is now made to FIG. 1 and similarly as in FIG. 3D where anon-volatile memory device 100 includes a memory element 120. The memoryelement 120 includes a conductive oxide layer 101, an ion impedinglayer, and an electrolytic tunnel barrier layer 105. The layers 101,103, and 105 of the memory element 120 are electrically in series withone another. Preferably, surfaces 101 b, 101 t, 103 t, and 105 t of thelayers 101, 103, and 105 are substantially planar surfaces or share thesame undulations and have substantially uniform thickness t₁, t₂, andt₃, respectively.

The conductive oxide layer 101 includes mobile ions 111 that are movablebetween the electrolytic tunnel barrier layer 105 and the conductiveoxide layer 101 in response to an electric field having a predeterminedmagnitude and direction, as will be described in greater detail below.The conductive oxide layer 101 can be a conductive perovskite. Examplesof conductive perovskites include but are not limited to PCMO, LNO,LCMO, LSCO, LSMO, PMO, strontium titanate (STO), and a reduced STO. Thethickness t₃ of the conductive oxide layer 101 will be applicationspecific. For example, an approximate range of thicknesses can be fromabout 100 Å to about 300 Å. As one example, the thickness t₃ can beapproximately 250 Å. The conductive oxide layer 101 can be formed usingmicroelectronics fabrication techniques that are well understood in thesemiconductor art for forming thin films. Example fabrication techniquesinclude but are not limited to atomic layer deposition (ALD), chemicalvapor deposition (CVD), sputtering, molecular beam epitaxy (MBE),spin-on deposition, pulsed laser deposition, electron-beam (e-beam)deposition, and thermal evaporation.

The ion impeding layer 103 is configured to substantially stop ionmovement between the electrolytic tunnel barrier layer 105 and theconductive oxide layer 101 when a voltage that is less than apredetermined magnitude is applied across the memory element 120, aswill be described in greater detail below. The material selected for theion impeding layer 103 will be application dependent. However, suitablematerials for the ion impeding layer 103 include but are not limited toLaAlO₃, TiO_(x), TaO_(x), AlO_(x), SiC, SiO_(x), IrO_(x), MgO, Pt,strontium ruthenate (SRO), and a reduced SRO.

Criteria for selecting the material for ion impeding layer 103 mayinclude but are not limited to a material operative as a mobilitybarrier to the mobile ions 111, a material having a high activationenergy for migration of mobile ions 111 to vacancy sites in thematerial, a material having stoichiometrically too few sites formigration of the mobile ions 111, and a material having a lowconductivity to the mobile ions 111 and having an electricalconductivity that is higher that an electrical conductivity of amaterial for the electrolytic tunnel barrier layer 105.

The electrolytic tunnel barrier layer 105 is made from an insulatingmaterial (e.g., a dielectric material) that allows ion movement. Thoseskilled in the art will appreciate that the term electronic refers toelectron or hole movement, while the term electrical or electrolyterefers to ion movement. Accordingly, an electrolytic tunnel barrier is amaterial with bulk properties of an electronic insulator that allowsionic movement but is thin enough to allow for electron tunneling.Suitable materials for the electrolytic tunnel barrier layer 105 includebut are not limited to yttria-stabilized zirconia (YSZ), ZrO₂, HfO₂, andEr₂O₃. The electrolytic tunnel barrier layer 105 is operative to provideelectron tunneling such that the memory element 120 has a non-linear I-Vcurve and the current flowing through the memory element 120 is anon-linear function of the voltage applied across the memory element120. Tunneling mechanism for the electrolytic tunnel barrier layer 105include but are not limited to single step tunneling processes (e.g.,direct tunneling, Fowler-Nordheim tunneling, and thermionic fieldemission tunneling) and multi-step tunneling processes (e.g.,trap-assisted tunneling).

The material and thickness t₁ for the electrolytic tunnel barrier layer105 will be application dependent. Preferably, the thickness t₁ of theelectrolytic tunnel barrier layer 105 is approximately 100 Å or less.More preferably, the thickness t₁ is approximately 50 Å or less. Forexample, the thickness t₁ can be approximately 25 Å. If the electrolytictunnel barrier layer 105 is too thick, tunneling may not occur or thevoltage across the memory element 120 necessary for tunneling may be toohigh. For example, currents generated by the applied voltage may exceedcurrent density limitations of the memory element and/or conductivearray lines, the resulting electric field generated by the appliedvoltage may exceed breakdown limits of the thin film materials in thememory element, or the magnitude of the applied voltage may requiredriver circuitry that exceeds an area budget for a memory design. Thethickness t₂ for the ion impeding layer 103 is approximately no greaterthan the thickness t₁ for the electrolytic tunnel barrier layer 105(e.g., t₂≦t₁). If the ion impeding layer 103 is too thick, devicecurrents may be too low and/or the mobile ions 111 may not be able totravel through the ion impeding layer 103. For example, the thickness t₂for the ion impeding layer 103 can be approximately 20 Å. As anotherexample, if the ion impeding layer 103 is made from silicon carbide(SiC), then the thickness t₂ can be approximately 10 Å. The ion impedinglayer 103 and the electrolytic tunnel barrier layer 105 may be formedusing the fabrication techniques described above for the conductiveoxide layer 101.

Referring again to FIG. 1, the memory element 120 stores data as aplurality of conductivity profiles (e.g., resistive states). One of theconductivity profiles may be indicative of a first resistive state(e.g., a logic 1 or an erased state) and another one of the conductivityprofiles may be indicative of a second resistive state (e.g., a logic 0or a programmed state). For example, in FIG. 1 the mobile ions 111 arepositioned in the conductive oxide layer 101 and the memory element 120can store data as the first conductivity profile (e.g., erased state orlogic 1). Turning now to FIG. 2A, a first write configuration 200includes a voltage source 201 operative to apply a first write voltageV_(W1) across the memory element 120. A switch 203 is connected with thevoltage source 201 and is operative to apply the first write voltageV_(W1) across the memory element 120. Conversely, when the switch 203 isopen the first write voltage V_(W1) is no longer applied across thememory element 120. As depicted in FIG. 2A, the switch 203 is closed sothat the first write voltage V_(W1) is applied across the memory element120 at nodes 202 and 204. A magnitude and polarity of the first writevoltage V_(W1) is operative to generate a first electric field E₁ havinga magnitude sufficient to cause a quantity of the mobile ions 111 tomove from the conductive oxide layer 101, through the ion impeding layer103, and into the electrolytic tunnel barrier layer 105. Those skilledin the art will appreciate that the first electric field E₁ has aplurality of magnitudes depending on the dielectric constant andconductivity of the specific materials being used for the memory element120. Based on the direction of the first electric field E₁ and on thedirection of movement of the mobile ions 111, the mobile ions 111depicted in FIG. 2A have a negative charge and move in a direction thatis opposite that of the first electric field E₁. For example, the mobileions 111 can be negatively charged oxygen ions (O⁻). However, the chargeof the mobile ions 111 is not limited to negatively charge species ofions and in some applications the mobile ions 111 may be positivelycharged ions.

Moving now to FIG. 2B, a quantity 211 of the mobile ions 111 have movedfrom the conductive oxide layer 101, through the ion impeding layer 103,and into the electrolytic tunnel barrier layer 105 after the firstelectric field E₁ was applied. Reference to a quantity may include someor all of the mobile ions 111. The switch 203 is opened and the firstwrite voltage V_(W1) is no longer applied across the memory element 120.The ion impeding layer 103 is operative to substantially stop (seedashed arrows 205) the quantity 211 from moving back through the ionimpeding layer 103 and into the conductive oxide layer 101 unless awrite voltage having a sufficient magnitude and polarity (e.g., a secondwrite voltage as will be described below) is applied across the memoryelement 120. In FIG. 2B, the voltage applied across the memory element120 is substantially 0V; however, as will be described below, a readvoltage having a magnitude that is less than the write voltage can beapplied across the memory element 120. The ion impeding layer 103 isfurther operative to substantially stop movement of the quantity 211when the read voltage is applied across nodes (202, 204). Moreover, whenthe memory element 120 is half-selected such that a half-select voltageis applied across the nodes (202, 204) the ion impeding layer 103 isalso operative to substantially stop movement of the quantity 211. Itshould be appreciated by those skilled in the art that the term“quantity” refers only to those ions that are impeded by the ionimpeding layer 103 and not any ions that may not be impeded.

The relocation of the mobile ions 111 in the conductive oxide layer 101to the electrolytic tunnel barrier layer 105 (i.e., quantity 211)results in a change in electrical conductivity of the memory element 120such that its conductivity profile is switched from the firstconductivity profile present in FIG. 1 to a second conductivity profilepresent in FIGS. 2B and 2C. Accordingly, the application of the firstwrite voltage V_(W1) has effectuated a writing of new data to the memoryelement 120 and the second conductivity profile is indicative of the newdata. In one embodiment, the second conductivity profile is indicativeof a logic 0 or a programmed state of the memory element 120.

Referring to FIG. 2C, in a configuration 220, the memory element 120 isdepicted with the quantity 211 still positioned in the electrolytictunnel barrier layer 105 such that the memory element stores data as thesecond conductivity profile. The voltage source 201, the switch 203, andtheir connection with nodes (202, 204) are not depicted. However, likethe configuration 200 depicted in FIG. 2B, the stored data (e.g., thesecond conductivity profile) is retained in the absence of power. Thememory element 120 may be one of a plurality of memory elements 120 in anon-volatile memory device, such as a removable memory device (e.g., aSD card or USB Thumb Drive). Therefore, the configuration 200 mayrepresent the non-volatile memory device when it is inserted into a hostsystem and the configuration 220 may represent the non-volatile memorydevice when it is removed from the host system. In either case, the ionimpeding layer 103 is operative to improve retention of stored data ineach of the plurality of memory elements 120 in the memory device.Consequently, data retention, that is, the ability of the memory element120 to retain stored data over a period of time in the absence of power,is enhanced by the ion impeding layer 103.

Turning now to FIG. 3A, a second write configuration 300 includes avoltage source 301 configured to apply a second write V_(W2) voltageacross the memory element 120 at nodes (202, 204). Prior to theapplication of the second write voltage V_(W2), the memory element 120stores data as the second conductivity profile. A switch 303 is closedand the second write voltage V_(W2) generates a second electric filed E₂having a magnitude sufficient to move the quantity 211 from theelectrolytic tunnel barrier layer 105, through the ion impeding layer103, and back into the conductive oxide layer 101. Based on thedirection of the second electric filed E₂ and the direction of movementof the quantity 211, the mobile ions have a negative charge.

Moving now to FIG. 3B, the switch 303 is open; however, the applicationof the second write voltage V_(W2) has reversibly switched theconductivity profile of the memory element 120 from the secondconductivity profile (e.g., logic 0 or programmed state) to the firstconductivity profile (e.g., logic 1 or erased state) and the quantity211 that was previously disposed in the electrolytic tunnel barrierlayer 105 has moved through the ion impeding layer 103 and into theconductive oxide layer 101. The mobile ions now reside in the conductiveoxide layer 101 and are denoted as quantity 311. The re-introduction ofthe quantity 311 back into the conductive oxide layer 101 changes theconductivity profile of the memory element 120. Consequently, theapplication of the second write voltage V_(W2) has effectuated a writingof new data to the memory element 120. The ion impeding layer 103 isoperative to substantially stop 305 the quantity 311 from moving backthrough the ion impeding layer 103 and into the electrolytic tunnelbarrier layer 105 when a voltage having a magnitude that is less thanthe first or second write voltages (V_(W1), V_(W2)) is applied acrossthe memory element 120. As was described above, the ion impeding layer103 is further operative to substantially stop ion movement across theion impeding layer 103 when the voltage applied across the memoryelement 120 is a read voltage or a half-select voltage.

Referring now to FIG. 3C, a configuration 320 depicts the memory elementwithout the power source 301. In the configuration 320, the ion impedinglayer 103 is operative to substantially stop ion motion such that thefirst conductivity profile is retained in the absence of power. As wasdescribed above, the configurations depicted in FIGS. 3B and 3C mayrepresent a non-volatile memory device when it is inserted and removedfrom a host system, respectively. Accordingly, the application of thefirst write voltage V_(W1), followed by the application of the secondwrite voltage V_(W2), has returned the memory element 120 to the firstconductivity profile depicted in FIG. 1. Although not depicted, are-application of the first write voltage V_(W1) to the configurationdepicted in FIG. 3C will reversibly switch the first conductivityprofile to the second conductivity profile depicted in FIGS. 2B and 2C.

Reference is now made to FIGS. 4A through 4D where a read voltage isapplied across the memory element 120 at nodes (202, 204). FIGS. 4A and4B depict a voltage source 401 for generating a read voltage V_(R1)having a first polarity and FIGS. 4C and 4D depict a voltage source 431for generating a read voltage V_(R2) having a second polarity that isopposite the first polarity. Regardless of read voltage polarity, amagnitude of the read voltage is less than the magnitude of the writevoltage (V_(W1), V_(W2)) in order to prevent previously stored data frombeing overwritten. For example, if the magnitude of the write voltages(V_(W1), V_(W2)) is approximately 4V, then the magnitude of the readvoltage can be approximately 1.5V. In some applications, the readvoltage will be applied with only one polarity. In other applications,the polarity of the read voltage may be alternated (e.g., +V_(R) and−V_(R)). For example, approximately half of the read operations areeffectuated using a first polarity and approximately half of the readoperations are effectuated using a second polarity.

Referring again to FIGS. 4A and 4B, in configurations 410 and 420, aswitch 403 connected with the voltage source 401 applies the first readvoltage V_(R1) across the memory element 120. As a result, an electricfield E_(R1) and a read current I_(R1) are generated. A magnitude of theread current I_(R1) is indicative of the value (i.e., resistive state)of data stored in the memory element 120. In FIG. 4A data is stored asthe second conductivity profile and in FIG. 4B data is stored as thefirst conductivity profile. Depending on the application, conventionssuch as logic 0 and logic 1, or programmed and erased, may be associatedwith the conductivity profiles. The ion impeding layer 103 is operativeto substantially stop (see dashed arrows 405) ion movement between theelectrolytic tunnel barrier layer 105 and the conductive oxide layer 101as depicted in FIG. 4A and to prevent ion movement from the conductiveoxide layer 101 and into the electrolytic tunnel barrier layer 105 asdepicted in FIG. 4B. Consequently, the first and second conductivityprofiles are not corrupted or disturbed by the application of the firstread voltage V_(R1).

Turning now to FIGS. 4C and 4D, the polarity of the read voltage V_(R2)is reversed. In configurations 430 and 440, a switch 433 is connectedwith a voltage source 431 that applies the second read voltage V_(R2)across the memory element 120. The read voltage V_(R2) generates anelectric field E_(R2) and a read current I_(R2) that are opposite indirection to the electric field E_(R1) and the read current I_(R1)depicted in FIGS. 4A and 4B. Nevertheless, the ion impeding layer 103 isoperative to substantially stop (see dashed arrows 405) ion movementfrom the electrolytic tunnel barrier layer 105 and back into theconductive oxide layer 101 as depicted in FIG. 4C where the memoryelement 120 stores data as the second conductivity profile. Similarly,ion impeding layer 103 is operative to substantially stop ion movementfrom the conductive oxide layer 101 and into the electrolytic tunnelbarrier layer 105 as depicted in FIG. 4D where the memory element 120stores data as the first conductivity profile. Consequently, the firstand second conductivity profiles are not corrupted or disturbed by theapplication of the second read voltage V_(R2).

Depending on the charge or ionization state of the mobile ions 111, thedirection of the electric field can enhance data retention. In FIGS. 4Aand 4D, assuming the mobile ions (211, 311) are negatively charged, theelectric fields (E_(R1), E_(R2)) are operative to displace the mobileions away from the ion impeding layer 103 thereby aiding the ionimpeding layer 103 in substantially stopping ion movement. In contrast,the electric fields (E_(R1), E_(R2)) in FIGS. 4B and 4C are operative todisplace the mobile ions (311, 211) towards the ion impeding layer 103.Accordingly, the ion impeding layer 103 must be configured tosubstantially stop the ion movement in the worst case scenario where thecharge of the ion species and the direction of the electric fielddisplace the mobile ions towards the ion impeding layer 103. Althoughthe above discussion focused on electric fields generated by readvoltages, the same principles apply when the applied voltage is ahalf-select voltage, because in both cases ion motion is substantiallystopped. On the other hand, in the case where the applied voltage is awrite voltage, ion movement is necessary to effectuate the switching ofthe conductivity profile of the memory element 120. Additionally, theion impeding layer 103 is operative to substantially stop ion movementof the quantity of mobile ions that may be caused by internal electricfields and concentration gradients caused by ion build-up in theelectrolytic tunnel barrier layer 105 and/or the conductive oxide layer101.

Referring again to FIGS. 4A through 4D, the ion impeding layer 103 isalso operative to substantially stop ion movement due to electrostaticcharge repulsion 409 between ions 211 or 311 as depicted by arrows 409.For example, ions that are in close proximity to one another and havingidentical charges will repel one another with varying amounts of force.Absent the ion impeding layer 103, the repelling force can cause some ofthe mobile ions 211 or 311 to move (e.g., drift) between the conductiveoxide layer 101 and the electrolytic tunnel barrier layer 105. Overtime, that movement of ions will increase or decrease the conductivityof the conductive oxide layer 101 and corrupt the value of stored datain the memory element 120. The mutual repulsion occurs even when novoltages are applied across the memory element 120.

Although the forgoing discussion has disclosed ions with negativeionization state, the ionization state of the ions is applicationdependent and the material selected for the memory element 120 caninclude materials configured to operate with ions having a positiveionization state.

Reference is now made to FIG. 5 where a configuration 500 includes apair of electrodes 501 and 503 that sandwich the memory element 120. Thememory element 120 is electrically in series with the pair of electrodes(501, 503). The electrode 501 is in contact with the electrolytic tunnelbarrier layer 105 and the electrode 503 is in contact with theconductive oxide layer 101. The aforementioned read, write, andhalf-select voltages can be applied across the memory element 120 byconnecting the voltage sources with the nodes (202, 204). The pair ofelectrodes (501, 503) may be made from an electrically conductivematerial including but not limited to a metal, a metal alloy, platinum(Pt), tungsten (W), aluminum (Al), and a conductive oxide material.Although not depicted in FIG. 5, additional thin film layers may bepositioned between the electrodes (501, 503) and the layers of thememory element 120. Those layers include but are not limited to gluelayers, diffusion barriers, adhesion layers, anti-reflection layers, andthe like. For example, an adhesion layer may be positioned between asurface 101 b of the conductive oxide layer 101 and the electrode 503 topromote adhesion between the materials of the electrode 503 and theconductive oxide layer 101. Similarly, a glue layer may be positionedbetween a surface 105 t of the electrolytic tunnel barrier layer 105 andthe electrode 501. In that the memory element 120 is electrically inseries with the pair of electrodes (501, 503) that sandwich it, thecombination forms a memory element 520 where voltages for dataoperations (e.g., read and write voltages) may be applied to nodes (202,204).

Moving now to FIGS. 6A and 6B, configurations 610 and 620 include anon-ohmic device 611 and 621 respectively. The non-ohmic devices 611 and621 are sandwiched between the pair of electrodes (501, 503) and areelectrically in series with the memory element 120 and the pair ofelectrodes (501, 503). As was discussed above, each memory element 120stores data as a plurality of conductivity profiles with discreteresistances at certain voltages. Therefore, each memory element 120 canbe schematically depicted as a resistor that is electrically in serieswith the non-ohmic devices 611 and 621. A resistance at a certainvoltage of a specific memory element 120 is indicative of a value ofstored data in that memory element 120. As an example, each memoryelement 120 can store a single bit of data as one of two distinctconductivity profiles having a first resistive state R₀ at a readvoltage V_(R) indicative of a logic “0” and a second resistive state R₁at V_(R) indicative of a logic “1”, where R₀≠R₁. Preferably, a change inconductivity, measured at the read voltage V_(R), between R₀ and R₁differs by a large enough factor so that a sense unit that iselectrically coupled with the memory element 120 can distinguish the R₀state from the R₁ state. For example, the factor can be at least afactor of approximately 5. Preferably, the predetermined factor isapproximately 10 or more (e.g., R₀≈1MΩ and R₁≈100 kΩ). The larger thepredetermined factor is, the easier it is to distinguish betweenresistive states R₀ and R₁. Furthermore, large predetermined factors mayalso allow intermediate resistive states (e.g., R₀₀, R₀₁, R₁₀, and R₁₁).

The resistance of the memory element 120 may not be a linear function ofthe voltage applied across the memory element 120 at the nodes (202,204). Therefore, a resistance R_(S) of the memory elements 120 canapproximately be a function of the applied voltage V such that R_(S)≈f(V). The applied voltage V can be a read voltage, a write voltage, or ahalf-select voltage. Moreover, because the non-ohmic devices 611 and 621are electrically in series with the memory element 120, a resultingseries resistance creates a voltage drop across the non-ohmic devices611 and 621 such that the actual voltage across the memory element 120will be less than the voltage applied across the nodes (202, 204). Asone example, if the read voltage V_(R)≈3V and the voltage drop acrossthe non-ohmic devices 611 and 621 is approximately 2.0V, then aneffective read voltage across the memory element 120 is approximately1.0V.

The non-ohmic devices 611 and 621 create a non-linear I-V characteristiccurve that falls within a desired operational current-voltage range fordata operations (e.g., read and write operations) to the memory element120. The non-ohmic devices 611 and 621 substantially reduce or eliminatecurrent flow when the memory element 120 is not selected for a read orwrite operation. The non-ohmic devices 611 and 621 allow data to bewritten to the memory element 120 when a write voltage V_(W) ofappropriate magnitude and polarity is applied across the nodes (202,204) of a selected memory element 120. Similarly, the non-ohmic devices611 and 621 allow data to be read from the memory element 120 when aread voltage V_(R) of appropriate magnitude and polarity is appliedacross the nodes (202, 204) of a selected memory element 120. Anadditional function of the non-ohmic devices 611 and 621 is tosubstantially reduce or eliminate current flow through half-selected andun-selected memory elements 120.

The non-ohmic devices 611 and 621 may include a plurality of layers ofthin film materials that are in contact with one another and are denotedas n in FIGS. 6A and 6B. Those layers can include a pair of electrodesthat sandwich one or more layers of a dielectric material. Thedielectric material(s) are operative as a tunnel barrier layer(s) thatgenerate the non-linear I-V characteristic of the non-ohmic devices 611and 621. As one example, the non-ohmic devices 611 and 621 can comprisea sandwich of Pt electrode/TiO_(x) dielectric layer/Pt electrode. Thethicknesses of the Pt and TiO_(x) materials will be applicationdependent. The Pt electrodes may have a thickness in a range from about500 Å to about 100 Å, for example. The TiO_(x) dielectric layer may havea thickness in a range from about 50 Å to about 20 Å, for example.Examples of suitable materials for the dielectric layers for thenon-ohmic devices 611 and 621 include but are not limited to SiO₂,Al₂O₃, SiN_(x), HfSiO_(x), ZrSiO_(x), Y₂O₃, Gd₂O₃, LaAlO₃, HfO₂, ZrO₂,Ta₂O₅, TiO_(x), yttria-stabilized zirconia (YSZ), Cr₂O₃, and BaZrO₃.Suitable materials for the electrically conductive layers for theelectrodes of the non-ohmic devices 611 and 621 include but are notlimited to metals (e.g., aluminum Al, platinum Pt, palladium Pd, iridiumIr, gold Au, copper Cu, tantalum Ta, tantalum nitride TaN, titanium(Ti), and tungsten W), metal alloys, refractory metals and their alloys,and semiconductors (e.g., silicon Si).

Alternatively, the non-ohmic devices (611, 621) can include a pair ofdiodes connected in a back-to-back configuration (not shown), forexample. Each of the diodes can be manufactured to only allow current toflow in a certain direction when its breakdown voltage (of apredetermined magnitude and polarity) is reached.

In FIG. 6A, the non-ohmic device 611 is positioned adjacent to electrode501; whereas, in FIG. 6B, the non-ohmic device 621 is positionedadjacent to electrode 503. In some applications, the material for thepair of electrodes (501, 503) will be compatible with the electrodematerial for the non-ohmic devices 611 and 621. In those applications,one of the pair of electrodes (501, 503) can serve as one of theelectrodes for the non-ohmic devices 611 and 621.

Reference is now made to FIG. 7A, where a non-volatile memory device 700includes a plurality of first conductive array lines 711 (one isdepicted) and a plurality of second conductive array lines 713 (one isdepicted), and a plurality memory plugs 702 (one is depicted). Eachmemory plug 702 includes a first terminal 701 in electricalcommunication with only one of the first conductive array lines 711 anda second terminal 703 in electrical communication with only one of thesecond conductive array lines 713. Each memory plug 702 includes amemory element 120 that is electrically in series with the first andsecond terminals (701, 703) and the layers 101, 103, and 105 of thememory element 120 are electrically in series with one another. Thefirst and second terminals (701, 703) can be the pair of electrodes(501, 503) described in reference to FIGS. 5, 6A, and 6B. As depicted inFIG. 7A, the memory plug 702 may include the above mentioned non-ohmicdevices, such as the device 611 or the device 613 (not shown). Thenon-ohmic device is electrically in series with the first and secondterminals (701, 703) and with the memory element 120. The position ofthe non-ohmic device in the memory plug 702 may be as depicted (e.g.,device 611) or the non-ohmic device can be positioned between the secondterminal 703 and the memory element 120. Although, non-ohmic device 611is depicted, the memory plug 702 need not include a non-ohmic device andthe first terminal 701 may be in contact with the memory element 120.

Although a coordinate system is not depicted, the first conductive arraylines 711 may be substantially aligned with a X-axis (e.g., running fromleft to right on the drawing sheet) and the second conductive arraylines 713 may be substantially aligned with a Y-axis (e.g., looking intothe drawing sheet). The aforementioned read and write and voltages areapplied to a selected memory plug 702 by applying the voltages acrossthe two conductive array lines that the memory plug 702 is positionedbetween. In FIG. 7A, by applying the read and write and voltages at thenodes (202, 204) stored data can be read from the selected memory plug702 or new data can be written to the selected memory plug 702. A readcurrent I_(R) flows through the selected memory plug 702, the memoryelement 120, and the non-ohmic device (611 or 613) if it is included inthe memory plug 702. The direction of flow of the read current I_(R)(e.g., substantially along a Z-axis) will depend on the polarity of theread voltage. For example, if a positive read voltage potential isapplied to the node 202 and a negative read voltage potential is appliedto the node 204, then the read current I_(R) will flow from the firstconductive array line 711 to the second conductive array lines 713. Insome applications, a memory cell 705, the repeatable unit that makes upthe array, may include all or a portion of the conductive array lines(711, 713) as denoted by the dashed line for the memory cell 705.

Turning now to FIG. 7B, schematic view of the non-volatile memory device700 includes the plurality of first and second conductive array lines(711, 713) and a plurality of the memory plugs 702 connected with theplurality of first and second conductive array lines (711, 713) by theirrespective first and second terminals (701, 703). The plurality of firstconductive array lines 711 are substantially aligned with the X-axis anddefine a row direction (row 731) and the plurality of second conductivearray lines 713 are substantially aligned with the Y-axis and define acolumn direction (col 733). Preferably, the first and second conductivearray lines (711, 713) are arranged substantially orthogonal to eachother. Conductive array lines 711′ and 713′ are selected array linesbecause a read or write voltage is applied to those lines at nodes (202,204) to select memory plug 702′ for a data operation (e.g., read orwrite operation).

In FIG. 7C, the non-volatile memory device 700 includes the selectedmemory plug 702′ positioned at the intersection of selected conductivearray lines 711′ and 713′. Memory plugs 702 that are only connected withone of the selected conductive array lines (711′ and 713′) are denotedas half-selected memory plugs 702 h. The remaining memory plugs 702 inthe memory device 700 are un-selected memory plugs 702 because thererespective first and second terminals (701, 703) are connected withconductive array lines that are not at a read or write voltagepotential. It should be noted that the memory plug 702 identified withdashed line 7A-7A is depicted in cross-sectional view in FIG. 7A. As wasdescribed above, the memory plugs 702 may or may not include a non-ohmicdevice.

Referring now to FIG. 8A, the non-volatile memory device 700 includes asubstrate 801 that includes active circuitry 803 that is fabricated onthe substrate 801. As one example, the substrate 801 can be a silicon(Si) wafer and the active circuitry can be microelectronic devicesformed on the substrate 801 using a CMOS fabrication process. The memoryplugs 702 and their respective conductive array lines (711, 713) can befabricated on top of the active circuitry 803 in the substrate 801.Those skilled in the art will appreciate that an inter-levelinterconnect structure (not shown) can electrically couple theconductive array lines (711, 713) with the active circuitry 803 whichmay include several metal layers. For example, vias can be used toelectrically couple the conductive array lines (711, 713) with theactive circuitry 803. The active circuitry 803 may include but is notlimited to address decoders, sense amps, memory controllers, databuffers, direct memory access (DMA) circuits, voltage sources forgenerating the read and write voltages, just to name a few. Activecircuits 810-818 can be configured to apply the select voltagepotentials (e.g., read and write voltage potentials) to selectedconductive array lines (711, 713). Moreover, active circuits coupledwith the conductive array lines (711, 713) can be used to sense the readcurrent I_(R) from selected memory elements 120 during a read operationand the sensed current can be processed to determine the conductivityprofiles (e.g., the resistive state) of the selected memory elements120. In some applications, it may be desirable to prevent un-selectedarray lines (711, 713) from floating. The some of the active circuitscan be configured to apply an un-select voltage potential (e.g.,approximately a ground potential) to the un-selected array lines (711,713). A dielectric material 811 (e.g., SiO₂) may be used where necessaryto provide electrical insulation between elements of the non-volatilememory device 700.

In FIG. 8B, a non-volatile memory device 820 includes a plurality ofnon-volatile memory arrays that are vertically stacked above one another(e.g., along the Z-axis) and are positioned above a substrate 821 thatincludes active circuitry 823. The non-volatile memory device 820includes vertically stacked memory layers A and B and may includeadditional memory layers up to an nth memory layer. The memory layers A,B, . . . through the nth layer can be electrically coupled with theactive circuitry 823 in the substrate 821 by an inter-level interconnectstructure as was described above. Layer A includes memory plugs 702 aand first and second conductive array lines (711 a, 713 a), Layer Bincludes memory plugs 702 b and first and second conductive array lines(711 b, 713 b), and if the nth layer is implemented, then the nth layerincludes memory plugs 702 n and first and second conductive array lines(711 n, 713 n). Dielectric materials 825 a, 825 b, and 825 n (e.g.,SiO₂) may be used where necessary to provide electrical insulationbetween elements of the non-volatile memory device 820. Active circuits840-852 can be configured to apply the select voltage potentials (e.g.,read and write voltage potentials) to selected conductive array lines(e.g., 711 a, b, . . . n, and 713 a, b, . . . n). As was describedabove, the active circuits can be used to sense the read current I_(R)from selected memory elements 120 during a read operation and can beconfigured to apply the un-select voltage potential to the un-selectedarray lines.

Turning to FIG. 9, a table depicts data loss in memory elements with andwithout the ion impeding layer 103. In memory elements without the ionimpeding layer 103, the structure comprises a layer of PCMO (e.g., aconductive oxide layer) and a layer of YSZ (e.g., an electrolytic tunnelbarrier layer) sandwiched between a pair of Pt electrodes. For the 25 Åthick YSZ, the erase and program slopes are −15.6 and 9.3 respectively.For the 30 Å thick YSZ, the erase and program slopes are −17.3 and 4.3respectively. In contrast, for the memory element including the ionimpeding layer 103, the structure comprises a layer of PCMO (e.g.,conductive oxide layer 101), a layer of SiO_(x) (e.g., the ion impedinglayer 103), and a layer of YSZ (e.g., the electrolytic tunnel barrierlayer 105) sandwiched between a pair of Pt electrodes (e.g., 501, 503).For the 4 Å, 8 Å, and 20 Å thick SiO_(x) layers, the values for theerase and program slopes are lower than those of the memory elementswithout the ion impeding layer 103 and those lower values are indicativeof improved data retention.

In FIG. 10, the above erase and program slope values for the memoryelements with and without the ion impeding layer 103 are averaged andplotted as percent of initial current loss per decade versus time. Plotsfor erase and program states of memory elements without the ion impedinglayer 103 are denoted as 1001 and 1003 respectively. Plots for erase andprogram states of memory elements 120 with the ion impeding layer 103are denoted as 1002 and 1004 respectively.

Reference is now made to FIG. 11, where an exemplary memory system 1100includes the aforementioned non-volatile two-terminal cross-point memoryarray 700 (array 700 hereinafter) and the plurality of first conductiveand second conductive traces denoted as 711 and 713, respectively. Thememory system 1100 also includes an address unit 1103 and a sense unit1105. The address unit 1103 receives an address ADDR, decodes theaddress, and based on the address, selects at least one of the pluralityof first conductive traces (denoted as 711′) and one of the plurality ofsecond conductive traces (denoted as 713′). The address unit 1103applies select voltage potentials (e.g., read or write voltages) to theselected first and second conductive traces 711′ and 713′. The addressunit 1103 also applies a non-select voltage potential to unselectedtraces 711 and 712. The sense unit 1105 senses one or more currentsflowing through one or more of the conductive traces. During a readoperation to the array 700, current sensed by the sense unit 1105 isindicative of stored data in a memory plug (not shown) positioned at anintersection of the selected first and second conductive traces 711′ and713′. A bus 1121 coupled with an address bus 1123 can be used tocommunicate the address ADDR to the address unit 1103. The sense unit1105 processes the one or more currents and at least one additionalsignal to generate a data signal DOUT that is indicative of the storeddata in the memory plug. In some embodiments, the sense unit 1105 maysense current flowing through a plurality of memory plugs and processesthose currents along with additional signals to generate a data signalDOUT for each of the plurality of memory plugs. A bus 1127 communicatesthe data signal DOUT to a data bus 1129. During a write operation to thearray 700, the address unit 1103 receives write data DIN to be writtento a memory plug specified by the address ADDR. A bus 1125 communicatesthe write data DIN from the data bus 1129 to the address unit 1103. Theaddress unit 1103 determines a magnitude and polarity of the selectvoltage potentials to be applied to the selected first and secondconductive traces 711′ and 713′ based on the value of the write dataDIN. For example, one magnitude and polarity can be used to write alogic “0” and a second magnitude and polarity can be used to write alogic “1”. In other embodiments, the memory system 1100 can includededicated circuitry that is separate from the address unit 1103 togenerate the select potentials and to determine the magnitude andpolarity of the select potentials.

One skilled in the art will appreciate that the memory system 1100 andits components (e.g., 1103 and 1105) can be electrically coupled withand controlled by an external system or device (e.g., a microprocessoror a memory controller). Optionally, the memory system 1100 can includeat least one control unit 1107 operative to coordinate and controloperation of the address and sense units 1103 and 1105 and any othercircuitry necessary for data operations (e.g., read and writeoperations) to the array 700. One or more signal lines 1109 and 1111 canelectrically couple the control unit 1107 with the address and senseunits 1103 and 1105. The control unit 1107 can be electrically coupledwith an external system (e.g., a microprocessor or a memory controller)through one or more signal lines 1113.

As was described above in reference to FIGS. 8A and 8B, one or more ofthe arrays 700 can be positioned over a substrate that includes activecircuitry and the active circuitry can be electrically coupled with thearray(s) 700 using an interconnect structure that couples signals fromthe active circuitry with the conductive array lines 711 and 713. InFIG. 11, the busses, signal lines, control signals, the address, sense,and control units 1103, 1105, and 1107 can comprise the active circuitryand its related interconnect, and can be fabricated on a substrate(e.g., a silicon wafer) using a microelectronics fabrication technology,such as CMOS, for example.

Reference is now made to FIG. 12, where an electrical system 1200includes a CPU 1201 that is electrically coupled 1204 with a bus 1202,an I/O unit 1207 that is electrically coupled 1210 with the bus 1202,and a storage unit 1205 that is electrically coupled 1208 with the bus1202. The I/O unit 1207 is electrically coupled 1212 to external sources(not shown) of input data and output data. The CPU 1201 can be any typeof processing unit including but not limited to a microprocessor (μP), amicro-controller (μC), and a digital signal processor (DSP), forexample. Via the bus 1202, the CPU 1201, and optionally the I/O unit1207, perform data operations (e.g., reading and writing data) on thestorage unit 1205. The storage unit 1205 stores at least a portion ofthe data in the aforementioned non-volatile two-terminal cross-pointarray as depicted in FIGS. 7A through 8B. Each memory array includes aplurality of the two-terminal memory elements 120. The configuration ofthe storage unit 1205 will be application specific. Exampleconfigurations include but are not limited to one or more single layernon-volatile two-terminal cross-point arrays and one or more verticallystacked non-volatile two-terminal cross-point arrays. In the electricalsystem 1200, data stored in the storage unit 1205 is retained in theabsence of electrical power. The CPU 1201 may include a memorycontroller (not shown) for controlling data operations to the storageunit 1205.

Alternatively, the electrical system 1200 may include the CPU 1201 andthe I/O unit 1207 coupled with the bus 1202, and a memory unit 1203 thatis directly coupled 1206 with the CPU 1201. The memory unit 1203 isconfigured to serve some or all of the memory needs of the CPU 1201. TheCPU 1201, and optionally the I/O unit 1207, executes data operations(e.g., reading and writing data) to the non-volatile memory unit 1203.The memory unit 1203 stores at least a portion of the data in theaforementioned non-volatile two-terminal cross-point array as depictedin FIGS. 7A through 8B. Each memory array includes a plurality of thetwo-terminal memory elements 120. The configuration of the memory unit1203 will be application specific. Example configurations include butare not limited to one or more single layer non-volatile two-terminalcross-point arrays and one or more vertically stacked non-volatiletwo-terminal cross-point arrays. In the electrical system 1200, datastored in the memory unit 1203 is retained in the absence of electricalpower. Data and program instructions for use by the CPU 1201 may bestored in the memory unit 1203. The CPU 1201 may include a memorycontroller (not shown) for controlling data operations to thenon-volatile memory unit 1205. The memory controller may be configuredfor direct memory access (DMA).

Although the invention has been described in its presently contemplatedbest mode, it is clear that it is susceptible to numerous modifications,modes of operation and embodiments, all within the ability and skill ofthose familiar with the art and without exercise of further inventiveactivity. Furthermore, although several embodiments of the presentinvention have been disclosed and illustrated herein, the invention isnot limited to the specific forms or arrangements of parts so describedand illustrated. The invention is only limited by the claims.Accordingly, that which is intended to be protected by Letters Patent isset forth in the claims and includes all variations and modificationsthat fall within the spirit and scope of the claim.

1. A non-volatile memory device, comprising: a memory element (ME)having exactly two terminals, the ME including electrically in serieswith the two terminals a conductive oxide layer including mobile ions,an ion impeding layer, and an electrolytic tunnel barrier layer, the MEis reversibly switchable between different conductivity profiles byapplying different write voltages across the two terminals, and the ionimpeding layer operative to substantially stop mobile ion movementbetween the electrolytic tunnel barrier layer and the conductive oxidelayer when voltages other than the different write voltages are appliedacross the two terminals.
 2. The non-volatile memory device as set forthin claim 1, wherein the different conductivity profiles of the ME arenon-destructively determined by applying a read voltage across the twoterminals.
 3. The non-volatile memory device as set forth in claim 1,wherein the ion impeding layer is operative to substantially stop ionmovement when a read voltage is applied across the two terminals.
 4. Thenon-volatile memory device as set forth in claim 1 and furthercomprising: a non-ohmic device sandwiched between a pair of electrodes,the non-ohmic device is electrically in series with the two terminalsand with the pair of electrodes.
 5. The non-volatile memory device asset forth in claim 1, wherein the conductive oxide layer comprises aconductive perovskite.
 6. The non-volatile memory device as set forth inclaim 5, wherein the conductive perovskite is a material selected fromthe group consisting of PCMO, LSCO, LNO, LCMO, PMO, LSMO, strontiumtitanate (STO), and a reduced STO.
 7. The non-volatile memory device asset forth in claim 1, wherein the electrolytic tunnel barrier layer hasa first conductivity and the ion impeding layer has a secondconductivity that is higher than the first conductivity.
 8. Thenon-volatile memory device as set forth in claim 1, wherein current flowthrough the ME is a non-linear function of a voltage applied across thetwo terminals.
 9. The non-volatile memory device as set forth in claim8, wherein the ME includes a non-linear I-V curve.
 10. The non-volatilememory device as set forth in claim 1, wherein the mobile ions comprisemobile oxygen ions.
 11. The non-volatile memory device as set forth inclaim 1, wherein the ion impeding layer is made from a material selectedfrom the group consisting of LaAlO₃, TiO_(x), TaO_(x), AlO_(x), SiO_(x),IrO_(x), MgO, Pt, strontium ruthenate (SRO), and a reduced SRO.
 12. Thenon-volatile memory device as set forth in claim 1, wherein theelectrolytic tunnel barrier layer is made from an electronicallyinsulating material.
 13. The non-volatile memory device as set forth inclaim 12, wherein the electronically insulating material is a materialselected from the group consisting of yttria-stabilized zirconia (YSZ),ZrO₂, HfO₂, and Er₂O₃.
 14. The non-volatile memory device as set forthin claim 1, wherein a conductivity profile of the ME is indicative of atleast one bit of stored data that is retained in an absence ofelectrical power.
 15. An electrical system, comprising: a bus; aprocessing unit in electrical communication with the bus; aninput/output (I/O) unit in electrical communication with the bus; and amemory unit in electrical communication with the processing unit, thememory unit including a substrate including active circuitry, aplurality of first conductive array lines, a plurality of secondconductive array lines, and a plurality of memory cells, each memorycell including a first terminal in electrical communication with onlyone of the plurality of first conductive array lines and a secondterminal in electrical communication with only one of the plurality ofsecond conductive array lines, the plurality of memory cells and theplurality of first and second conductive array lines are positioned overthe substrate with the plurality of first and second conductive arraylines in electrical communication with at least a portion of the activecircuitry, the portion configured for data operations on the memorycells, each memory cell including a memory element (ME) electrically inseries with its respective first and second terminals, each ME havingexactly two electrodes, current flow through the ME is a non-linearfunction of a voltage applied across the two electrodes, the MEincluding electrically in series with its two electrodes a conductiveoxide layer including mobile ions, an ion impeding layer, and anelectrolytic tunnel barrier layer.
 16. The electrical system of claim15, wherein the memory unit includes a plurality of stacked non-volatiletwo-terminal cross-point memory arrays.
 17. A non-volatile memoryelement, comprising: a first terminal; a second terminal; a conductiveoxide layer including mobile ions; an electrolytic tunnel barrier layerhaving a first thickness that is approximately 50 Å or less, theelectrolytic tunnel barrier layer is permeable to the mobile ions when awrite voltage is applied across the first and second terminals; and anion impeding layer configured to substantially stop mobile ion movementbetween the conductive oxide layer and the electrolytic tunnel barrierlayer for voltages other than the write voltage that are applied acrossthe first and second terminals, and wherein the conductive oxide layer,the ion impeding layer, and the electrolytic tunnel barrier layer areelectrically in series with the first and second terminals.
 18. Thenon-volatile memory element as set forth in claim 17, wherein theconductive oxide layer comprises a conductive perovskite.
 19. Thenon-volatile memory element as set forth in claim 18, wherein theconductive perovskite is a material selected from the group consistingof PCMO, LSCO, LNO, LCMO, PMO, LSMO, strontium titanate (STO), and areduced STO.
 20. The non-volatile memory element as set forth in claim17, wherein the ion impeding layer is made from a material selected fromthe group consisting of LaAlO₃, TiO_(x), TaO_(x), AlO_(x), SiO_(x),IrO_(x), MgO, Pt, strontium ruthenate (SRO), and a reduced SRO.
 21. Thenon-volatile memory element as set forth in claim 17, wherein currentflow through the ME is a non-linear function of a voltage applied acrossthe first and second terminals.
 22. The non-volatile memory element asset forth in claim 17, wherein the electrolytic tunnel barrier layer ismade from an electronically insulating material selected from the groupconsisting of yttria-stabilized zirconia (YSZ), ZrO₂, HfO₂, and Er₂O₃.23. An electrical system, comprising: a bus; a processing unit inelectrical communication with the bus; an input/output (I/O) unit inelectrical communication with the bus; and a storage unit in electricalcommunication with the bus, the storage unit including a substrateincluding active circuitry, a plurality of first conductive array lines,a plurality of second conductive array lines, and a plurality of memorycells, each memory cell including a first terminal in electricalcommunication with only one of the plurality of first conductive arraylines and a second terminal in electrical communication with only one ofthe plurality of second conductive array lines, the plurality of memorycells and the plurality of first and second conductive array lines arepositioned over the substrate with the plurality of first and secondconductive array lines in electrical communication with at least aportion of the active circuitry, the portion configured for dataoperations on the memory cells, each memory cell including a memoryelement (ME) electrically in series with its respective first and secondterminals, each ME having exactly two electrodes and includingelectrically in series with its two electrodes a conductive oxide layerincluding mobile ions; an electrolytic tunnel barrier layer that ispermeable to the mobile ions when a write voltage is applied across thefirst and second terminals; and an ion impeding layer configured tosubstantially stop mobile ion movement between the conductive oxidelayer and the electrolytic tunnel barrier layer for voltages other thanthe write voltage that are applied across the first and secondterminals.
 24. A non-volatile memory device, comprising: a substrateincluding active circuitry; a plurality of first conductive array lines;a plurality of second conductive array lines; and a plurality of memorycells, each memory cell including a first terminal in electricalcommunication with only one of the plurality of first conductive arraylines and a second terminal in electrical communication with only one ofthe plurality of second conductive array lines, the plurality of memorycells and the plurality of first and second conductive array lines arepositioned over the substrate with the plurality of first and secondconductive array lines in electrical communication with at least aportion of the active circuitry, the portion configured for dataoperations on the memory cells, each memory cell including a memoryelement (ME) electrically in series with its respective first and secondterminals, each ME having exactly two electrodes and includingelectrically in series with its two electrodes a conductive oxide layerincluding mobile ions; an electrolytic tunnel barrier layer that ispermeable to the mobile ions when a write voltage is applied across thefirst and second terminals; and an ion impeding layer configured tosubstantially stop mobile ion movement between the conductive oxidelayer and the electrolytic tunnel barrier layer for voltages other thanthe write voltage that are applied across the first and secondterminals.
 25. The non-volatile memory device as set forth in claim 24,wherein the electrolytic tunnel barrier layer has a first thickness thatis approximately 50 Å or less.
 26. The non-volatile memory device as setforth in claim 25, wherein current flow through the ME is a non-linearfunction of a voltage applied across the two electrodes.
 27. Thenon-volatile memory device as set forth in claim 24, wherein the ionimpeding layer is made from a material selected from the groupconsisting of LaAlO₃, TiO_(x), TaO_(x), AlO_(x), SiO_(x), IrO_(x), MgO,Pt, strontium ruthenate (SRO), and a reduced SRO.
 28. The non-volatilememory device as set forth in claim 24, wherein the plurality of firstconductive array lines have an orientation that is substantiallyorthogonal to the plurality of second conductive array lines and eachmemory cell is positioned substantially between an intersection of oneof the plurality of first conductive array lines with one of theplurality of second conductive array lines.